Remote Sensing Final Review

Why is remote sensing important for Earth Science?

cost effective, non obtrusive, uniform method, global scale capability

Convert frequencies into wavelengths and name the spectral region

88.5 MHz, 3.39 m wavelength, radio FM 3 10^13 Hz, 10 micrometer, Infra-red 6 GHz, 5 cm

Using Wein's Law for two black body objects with a temperature of 1000 K (fire) and 390 K (background), at what central wavelengths are the two objects radiating?

2.8 micrometer for 1000 K and 10 micrometer at 290 K

Should a fire (T=1000 K) emit more radiation at 5 micrometer wavelength or at 15 micrometer wavelength? Describe how we could use this to detect fires?

A fire (1000K) will emit more radiation at 5 micrometer than at 15 micrometer, o to detect a fire we compare that land reflectance at 5 and 15 micrometer: fires should appear brighter at 5 micronmeter wavelength

According to the blackbody radiation theory, rank the following colors from highest temperature to lowest temperature

Blue, yellow, orange, red

List two forms of remote sensing that do not require the use of electromagnetic waves bouncing off the Earth's surface

Time- variable gravity remote sensing of the Earth, ex. with the Gravity Recovery and Climate Experiment mission named GRACE, measures the distance between two satellites but does not use EM waves to make them bounce off the Earth's surface

How do we call the bending of electromagnetic waves as it travels through the atmosphere through layers of different density

Refraction

When atmospheric particles are smaller than the wavelength of an electromagnetic wave, what happens to the energy of the electromagnetic wave?

The electromagnetic wave undergoes Rayleigh Scattering

What are the wavelengths of visible light?

0.4 micrometer to 0.4 micrometer

Over which part of the spectrum does the sun radiate 41% of its energy?

Visible part of the spectrum (0.4 to 0.7 micrometer)

What is a blackbody and how does it respond to incoming solar radiation?

A blackbody is not reflecting nor transmitting any income solar illumination, it absorbs all the incoming short wave radiation and re-emits the absorbed energy as long wave radiation.

Give 3 reasons why Landsat TM is vastly improved over Landsat MSS

Higher spatial resolution, higher data rate, higher radiometric resolution.

An 8-bit pixel takes up one byte of computer space; one kilobyte is 1024 bytes; one megabyte is 1024 kilobyte. How large is a Landsat Thematic Mapper image

in Megabytes knowing that each image includes 7 bands of 6,000 columns x 6,000 rows coded on an 8-bit pixel per band?

246 Mbyte

What is the range of pixel values for satellite data coded on 10 bits?

0-1023

What absorbs solar irradiance in functioning leaves?

Plant pigments (Chlorophyll a, b, Carotenes) and water absorbs the incoming radiation in the red and blue wavelengths principally.

When a leaf is damaged and is not functioning well, what happens to its reflectivity?

Its reflectivity decreases in the NIR but increases in the visible.

What is the effect of liquid water inclusion in a leaf on its reflectivity in the near IR?

The reflectivity in NIR decreases when the water content increases, especially near the absorption bands of water.

What is NDVI if R_nir is the reflectivity in NIR, R_red is the reflectivity in red, and R_green is the reflectivity in green?

NDVI = ( R_nir - R_red)/(R_nir + R_red)

93% of the electromagnetic radiation is emitted in which part of the electromagnetic spectrum?

Visible and near infra-red

What are the three dips in reflectance in the spectrum of a leaf at wavelengths of 1.45, 1.94 and 2.7 micron?

These are the water absorption bands.

What spectral band allows you to distinguish between clouds and snow and why?

Thermal Infra red because the reflectivity of snow decreases significantly in infra red whereas that of cloud stays high

What is the LUE of a plant and how does it relate to primary production?

The LUE is a photochemical index which relates to the amount of carbon dioxide uptake by vegetation per unit of energy absorbed. If the amount of solar radiation absorbed by the vegetation is known, then we can calculate the uptake of CO2 by the vegetation.

Why is it important to query about the temporal resolution of satellite data?

It will tell us how frequently we can get observations from the same patch of land or ocean.

What are the two absorption peaks of Chlorophyll A?

At about 450 nm and 650 nm.

When the concentration of chlorophyll-a increases in seawater, at which wavelength are the changes in radiance exiting the seawater the most pronounced?

Blue channel, at 443 nm

What does it mean "an empirical algorithm is used to infer chlorophypll content from the reflectance ratio of water at two wavelengths?"

An empirical relationship between chlorophyll concentration and the ratio of reflectances at two different wavelengths is found by comparing remote sensing data and in situ measurements of chlorophyll; and the regression curve obtained between these quantities is convert reflectance ratios to chlorophyll concentration in other places.

The following sensors can be used to detect chlorophyll concentration in the ocean

CZCS (Costal Zone Color Scanner) 1979-1986, SeaWiFS 1997-2011, MODIS 2000-present.

What is the black pixel used for in SeaWiFS?

The black pixel assumption is to assume that water leaving radiance in the near infra red is negligible, and this is used to estimate the contribution from scattering by aerosols.

When the phytoplankton is stressed, for instance due to a decrease in nutrient availability, what happens to the energy it receives from the sun?

The phytoplankton releases additional energy via fluorescence.

What is CDOM in ocean color remote sensing?

Colored Dissolved Organic Material

What is the difference between chlorophyll concentration and primary production?

Primary production is related to the GROWTH in chlorophyll concentration, not the absolute chlorophyll content.

As climate warms up, what will happen to phytoplankton production based on the satellite record from 1998 to 2008?

Phytoplankton production will decline because of enhanced stratification of the ocean waters caused by warmer surface temperature.

What visible wavelengths provide most information about suspended sediments in seawater?

0.58 to 0.69 micro meter

In the Hershel experiment that revealed "caloric rays", why is the temperature recorded in the red part of the spectrum higher than that in the blue part of the spectrum after dispersion by the prism?

The index of refraction of the prism varies with the wavelength and tends to concentrate rays in the infrared part of the spectrum more than in the blue part of the spectrum.

The thermal infrared emissivity of snow is almost 100% yet snow is highly reflective of solar radiation, why is that?

Snow is a poor reflector of thermal infra-red radiation, hence is a strong emitter at those wavelengths.

With what precision can MODIS estimate land surface temperature?

1 K

Using Wien's law, calculate the wavelength of the peak radiation of a fire pixel at 800 K? How would you detect a fire pixel using two bands, one at 3.6 micrometer and a second band at 11 micrometer?

The peak radiation is at 3.6 micrometer. A fire pixel has a radiant temperature > 310 K at 3.6 micrometer and 10 K larger than the radiance temperature measured at 11 micrometer.

What is a prograde orbit and what is the general direction of the satellite subtrack on the ground on an ascending orbit?

A prograde orbit has an inclination of less than 90 degrees, and the ground track varies from southwest to northeast during an ascending orbit.

At 570 km altitude, what is the typical speed of a satellite and how long does it take to orbit the Earth?

The satellite moves at 7.6 km/s and goes around the Earth in 1 and a half hour

Whether the orbit is prograde or retrograde, the successive orbits migrate westward longitudinally due to the oblateness of the Earth.

True

To get a sun-synchronous orbit, what does the orbit inclination need to be and is this a retrograde or prograde orbit?

The orbit needs to be retrograde with an inclination between 98 and 102 degree for altitudes between 700 and 1,500 km.

What is better for detecting a smoldering fire? A sensor capable of detecting temperature between 0 and 1,000 K with data coded on 8 bits or a sensor operating between 200K and 600 K with data coded on 8 bits?

Sensor between 200-600 K discriminates 1.6 K

When I look at the Earth from the North, with a satellite along a retrograde orbit, which way is the satellite moving?

Clockwise direction

The orbit of NASA's TRMM Tropical Rain Mapping Mission was 35 degrees, with a 46-day repeat cycle. Was this a prograde or retrograde orbit? Is it a sun-synchronous orbit? How far north and south can the satellite see rain? What is the point of such a long repeat cycle?

The orbit is prograde, it is not sun synchronous, data can only be collected up to 35 degree latitude north and south; the long repeat is ok because the satellite can observe rain at different times of the day.

What is the difference between layover and foreshortening?

Layover is an extreme case of foreshortening where ground layers fall on top of each other and become inverted.

What are the range and azimuth resolutions of a SAR system with a bandwith B = 10 MHz, a antenna length of L = 15 m and a a radar wavelength of 6 cm?

Range resolution is 15 m, azimuth resolution is 7.5 m.

The interferometric phase difference varies as:

δφ = 4 π/λB cos (θ-α);

where B = 60m, α= 45 deg, λ = 5.66 cm, and the incidence angle θ varies from 35 degrees in the near range to 38 degrees in the far range.

How many fringes (or full 360-deg cycles of δφ) will you see in the flat-earth interferogram going from near range to far range?

16.4 cycles

Give one example of interferometric application that works best with simultaneous baseline, i.e. two antennas flying on the same satellite.

Radar topographic mapping

Give an example of application of satellite radar interferometry with repeat track baseline, i.e. we combine data acquired by the same satellite, along the same track, at different times.

Detection of land motion.

How does synthetic aperture radar (SAR) generate high spatial resolution along the track of the satellite

SAR is a processing technique that integrates the signal received from a target on the ground over the entire time the target is within the beam of illumination of the radar.

I would like to examine radar returns from below a surface of dry sand, what wavelength should I use? C-band or L-band?

L-band is a long wavelength which is better for penetration in dry sand.

How long was the radar boom of SRTM radar and how many days did the Space Shuttle stay in space?

The boom is 60 m long and the Shuttle stayed 11 days in space.

If I use satellite radar interferometry using one repeat track along a given orbit, is it possible for me to completely characterize the vector motion of the surface?

Interferometric SAR only measures ground motion in the direction of illumination of the radar, which we call across track

What does it mean to remove the flat Earth from an interferogram?

We remove the interferometric fringes associated with the imaging of a flat Earth at that location.

I use data from an L-band (wavelength is 24 cm) synthetic aperture radar satellite interferometer with a 15-meter pixel sampling, a repeat pass cycle of 8 days, and an interferometric baseline of 50 m. I will use the data interferometrically to measure ground motion. What level of precision in ground motion should I be able to achieve with this instrument?

Over 8 days, I should be able to measure ground deformation of the order of 1/72th of the wavelength or 3 mm total displacement.

Considering radar reflections from a nearly flat ocean surface with a look angle of a couple of degrees, what kind of scattering will dominate?

Facets scattering

How does the backscattering cross section of the ocean change when the ocean surface is roughened by wind?

it depends on the look angle. If pointing at nadir, the backscattering cross section decreases; if pointing off nadir, the backscattering cross section increases.

Considering the radar altimeter waveform from an ocean surface, what is the leading edge and how is it affected by waves?

The leading edge is the first portion of the waveform where the signal amplitude rises, and as the surface gets rougher the leading edge is less steep, the rise in the signal amplitude is more gradual.

What is the typical footprint of the ERS-1 radar altimeter on the ocean surface?

1.7 km

Why do radar altimeters operate simultaneously at 2 frequencies?

Two frequencies are used to remove dispersive perturbations of the signal by the ionosphere

The measurement precision of radar altimeters for a single track is about what value?

2-3 cm for ERS-2 and TOPEX.

What are the fundamental differences between SST measured with IR and with microwaves?

IR is affected by clouds, microwave is not

What is the physical process that controls sea surface salinity?

SSS is most sensitive to evaporation and precipitation.

What is the name of the NASA mission that measured salinity from 2011 to 2015?

Aquarius

What are the advantages of IR-derived SST versus microwave-derived SST?

IR-derived SST has better spatial resolution, better precision and is not biased by surface winds.

Which of these functions radar altimeters do not perform?

Measure the height of subsea mountains.

With what precision do we measure the distance between the two GRACE satellites Tom and Jerry?

1 micrometer

What does it mean that GRACE has no vertical resolution?

GRACE cannot distinguish between a change in mass taking place in the mantle, the crust, the near subsurface, the surface or above ground.

What is the GRACE signal so strong in northern Canada?

GRACE picks up the postglacial rebound caused by the former Laurentide ice sheet.

GRACE can measure changes in mass in a unique way, that no other sensor can do.

GRACE can measure changes in terrestrial water storage like no other sensor can do.

In 2010, the rate of sea level rise went through a slump, global sea level even went down. GRACE explained the source of that signal. What was it?

There was an increase in land precipitation, in particular in Australia and northern south America where land water got locked for a while.

What is the definition of the geoid?

The geoid is the surface assumed by an undisturbed top of the sea, and visualized as continuing under the continents by water filling small frictionless channels.

What is the typical spatial resolution of the GRACE data and the typical temporal resolution of the data?

The spatial resolution is about 330 km and the temporal resolution is about 1 month

How many Gigatons of water is a drop of 1.5 cm/year of water over a circle of diameter equal to 330 km?

- About 1.3 Gt/yr

- About 1.3 cubic km per year

(not this one) About 1282 Gt/yr

maybe 1.3 GT/yr?

What is the nominal precision of GRACE in detecting changes in water mass near the surface of the Earth?

1 cm of water over a 300 km disk

Why do we sometimes say that GRACE is the only remote sensing mission that does not use electromagnetic waves; and yet this is not quite correct.

GRACE detects changes in the distance between two satellites instead of radiating electromagnetic waves to the Earth or measuring electromagnetic radiation from the Earth; BUT it uses electromagnetic waves to measure the inter-satellite distance.

What is an application that GRACE cannot address?

What is the total volume of water stored in an aquifer?

The total incoming solar radiation at the top of the atmosphere is 342 W/m2. How much radiation is reflected by clouds, aerosols and atmosphere? How much radiation is reflected by the surface? Deduce the albedo of the Earth from those 3 numbers.

Clouds, Aerosols, Atmosphere reflect 77 W/m2, Earth surface is 30 W/m2. Albedo is (77+30)/342 = 31%

The sea ice extent decreased from 15.5 million square km in 1978 to 14.0 million square km in 2007. Assume an incoming solar irradiation of 168 W/m2 at the surface, a sea ice albedo of 0.9 and an ocean albedo of 0.06. What is the percentage change in absorbed energy by the original 15.5 million square km as a result of the sea ice retreat?

In 1978, the surface absorbed (1-0.9)15.5e12168 = 260.4 e+12 W; In 2007, it was (1-0.9)14.e12168. + (1-0.06)1.5e12168. = 472 e+12 W, so the change is +81% more solar irradiation absorbed.

Name three factors that contribute to deforestation in the tropics

Government incentives, logging companies looking for profitable business, pulp companies.

In which part of the World was deforestation the most intense in 1990-1997?

Southeast Asia

Compare the rates of area lost per year in sea ice cover and tropical forest: Sea ice lost 1.5 million square km in 30 years, Forests lose 240 x 240 km = 57,600 square kilometer per year.

Deforestation is faster, but not that much, they are pretty much comparable numbers.

What is happening to the albedo of Greenland since year 2000? and which satellites/sensors can be used to study this phenomenon?

The albedo of Greenland is decreasing on average, especially along the coastal areas, but also in the interior as a result of melt, snow metamorphism, dust deposition, and this is clearly visible in time series of land albedo measured by MODIS.

What is the effect of deforestation on land temperature in the tropics versus in the boreal forests according to the paper published in PNAS by Bala et al. in 2007?

Deforestation in the tropics warms up the climate, deforestation in the boreal forests cools down the climate.

What is the average rate of precipitation on Earth and where is 2/3 of the precipitation falling?

1 m/yr and 2/3 in tropical regions.

How do we measure rainfall with passive microwave and with active microwave?

With active microwave, we measure the backscatter from falling rain drops. With passive microwave, we measure the thermal emission from the atmosphere.

Why is it easier to measure rainfall over the ocean with passive microwave?

Because the ocean is cold, rainfall is warm, so there is a strong change in brightness temperature.

Why do we operate LiDAR with green light instead of near infra red?

Green light offers greater penetration in water (coastal mapping) and wet atmosphere.

When considering LiDAR returns from a forest canopy, what is the intensity of the LiDAR return related to?

The return intensity depends on the vertical distribution of vegetation biomass.

Why does ICESat-2 use 3 beams to sample the ice sheet surface instead of just one?

The 3 pairs of beams provide information about surface slope in the cross track dimension which is needed to interpret where the return signal comes from.

Why does CALIOP use two different lasers and two different polarizations to study clouds?

The different polarizations are used to separate different shape scattering objects (oblong drops versus spherical drops); the different laser frequencies are used to separate different size scatterers (small water droplets versus aerosol particulates).

Does Lidar work at night?

Yes. In fact the sunlight is competing with the Lidar.

How deep can Lidar penetrate in seawater to map bathymetry?

About 10 m

Why is the Lidar precision about 2 cm but the accuracy is 14 cm?

The accuracy reflects how close the measurement corresponds to its true value; the precision refers to how much variability you observe from one measurement to the next over the same spot, i.e. the noise level around that data point

Based on our discussion would you rather use radar technology to drive a car autonomously or would require Lidar technology?

Tesla's radar technology may be winning the game over Google's lidar technology and there are a number of reasons for that.

How long does it take for a photon to do a round trip from the ICESAT-2 satellite to Earth and back to the satellite?

About 3 milliseconds

what does the elliptical orbit of the satellite form, which passes through the earth's center of gravity?

orbital frame

The _____ (y) of the orbit is the angle between Earth's equatorial plan and the orbital plane in ascending node

inclination

inclination angles <90 degrees

prograde orbit

inclination angles >90 degrees

retrograde orbit